Overview of Nanoporous Materials
There exists a wealth of inorganic nanoporous materials. Nanoporous materials can be unordered, such as pillared clays and silica gels, or ordered. The ordered porous materials comprise zeolites and zeolite like materials, ordered mesoporous materials and hierarchical materials presenting more than one level of porosity and structural order. Among the ordered nanoporous materials, ordered microporous and mesoporous materials attract a lot of attention by the materials science community (ref. 1). Ordered microporous and mesoporous materials can be described in terms of a host structure, which defines a pore structure, which may contain guest species. The voids between the linked atoms have a free volume larger than a sphere with a 0.25 nm diameter. Pores with free diameters of less than 2 nm are called micropores, and those in the range of 2 to 50 nm mesopores (ref. 2).
When the atoms of the host as well as the voids are arranged periodically with long-range order (at least 10 repeats in all directions) the materials produce sharp maxima in a diffraction experiment. These materials are crystalline. Zeolites and zeolite like materials are such crystalline materials based on a 3-dimensional 4 connected inorganic frameworks. Generally they contain silicon, aluminum and oxygen in their framework and exchangeable cations, water and/or other guest molecules within their pores. The framework structure may contain linked cages, cavities or channels, which are of the right size to allow small molecules to enter. The limiting pore sizes are roughly between 0.3 and 1 nm in diameter. Because of their unique porous properties, zeolites are used in a variety of applications with a global market of several million tons per annum. Major zeolite uses are in catalytic petrochemical processes such as cracking, hydrocracking, alkylation, isomerization, in cation-exchange (water softening and water purification), and in molecular separations and purification of gases and solvents.
Another class of nanoporous materials displays short range order only. They are amorphous with respect to diffraction experiments. When such materials are ordered, the pores are of uniform size with long-range order and produce diffraction maxima at d-values reflecting the pore-to-pore distance. In such ordered mesoporous materials, the pore structure is “crystalline”. Examples include MCM-41 (ref. 3), MCM-48 (ref. 4), SBA-15 (ref. 5), FSM-16 (ref. 6), TUD-1 (ref. 7).
Next to crystalline microporous materials and ordered mesoporous materials both having only one type of pores, poly-porous materials exist. A distinction can be made according to the ordering of the different types of porosity. Mesoporous zeolites having a secondary porosity composed of irregular mesopores in addition to the micropores are e.g. applied as Fluid Catalytic Cracking catalysts. The ultrastable Y zeolite as a result of dealumination and partial structure degradation presents intracrystalline mesopores in addition to micropores (ref. 8). The presence of mesopores is important in catalytic applications. These larger pores facilitate the diffusion of larger molecules into the interior of the zeolite crystals.
Mesoporous zeolites can be obtained using the replica technique, whereby the zeolite is grown within the pores of various kinds of carbon template (ref. 9-11). Another approach for building poly-porous materials involves the linking of microporous building units into materials that are ordered at the mesoscale. Zeotiles are examples of such hierarchical materials (ref. 12 and 13). Zeotiles are robust materials capable of withstanding higher temperatures and combine the advantages of both micro and mesoporous materials (ref. 12 and 13).
Functionalization of Nanoporous Materials
For application in catalysis, nanoporous materials need to be functionalized. In silicate frameworks, the introduction of trivalent elements such as aluminum, gallium or iron (but not limited to these elements) generates positive charge deficiencies in the oxide framework that needs to be compensated by cations in the pores. Charge compensation with a proton generates Brönsted acidity in the material.
The conversion of an as-synthesized zeolite into a Brönsted acid zeolite can be obtained via different routes. When the lattice charge is compensated by organic cations, calcination to remove the organic molecules converts the as-synthesized zeolite into an acid zeolite. Alkali and alkaline earth metal cation can be replaced with protons through cation exchange with an ammonium salt and deammoniation via heating, or else through contacting of the zeolite with diluted acid. Cation exchange with multivalent cations such as La3+ followed by calcination is another method known in the art to generate Brönsted acid sites in a zeolite.
Nanoporous materials often have insufficient Brönsted acidity for a desired catalytic application. Many kinds of modifications have been reported to enhance the Brönsted acidity and the catalytic activity. Other modifications aim at altering the porosity and surface chemistry.
Post synthesis modifications of zeolites include a lot of techniques to further control the acidity or the shape selectivity of a specific zeolite structure. There are three main types of post synthesis modification which can be applied to a zeolite (ref. 14): (i) structural modification in which the framework SiO2/M2O3 molar ratio (where M=Al or another trivalent metal cation) is altered resulting in a change in acidity; (ii) modification of the surface of the zeolite crystal to adapt the size of the pore opening; (iii) internal pore modification which block or alter the acid sites or restrict the internal pore diameter. If the material is meant for catalytic application then it has to maintain a sufficient structural integrity.
There are several ways to decrease the aluminum content of a zeolite, e.g. by steaming, acid leaching or contacting the zeolite with an aluminum complexing agent such as ethylene diamine tetra acetate.
For enhancing the content of trivalent elements such as Al3+ there are several options. In the prior art, many reports primarily deal with the alumination of zeolites, but the person skilled in the art understands that other trivalent elements can be incorporated in nanoporous materials in a similar manner. The incorporation of trivalent elements directly during the crystallization of the zeolite is sometimes difficult. In the so-called high silica zeolites, the acid catalytic activity is limited because of the small percentage of trivalent elements, particularly aluminum, incorporated in the zeolite framework during crystallization. Imparting greater activity for those zeolites can be achieved by inserting trivalent elements, particularly aluminum atoms into the framework in a post-synthesis operation. Several methods have been used to incorporate trivalent elements such as aluminum or gallium into high silica zeolites (ref. 14). A first method is the hydrothermal treatment of the zeolite material with aqueous aluminate solution. The zeolite to be aluminated is then treated with an aqueous solution of aluminate salt under hydrothermal conditions. The aluminate reacts with the silanol groups of a hydroxyl defect site inside the zeolite channels. Sometimes an aluminum atom is inserted into a tetrahedral vacancy where it substitutes for a framework silicon atom. For example, ZSM 5 zeolite with enhanced n-hexane cracking activity can be obtained by treating essentially inactive high silica ZSM 5 zeolite with sodium aluminate solution in an autoclave (ref 15).
Treatment with Aluminum halide vapors is another proven method for aluminum incorporation into zeolites. It has been confirmed by 27Al magic angle spinning NMR and IR spectroscopy that when highly siliceous zeolite such as Silicalite is treated with AlCl3 vapor at elevated temperature (500-600° C.), aluminum atoms take the position of silicons in the zeolite framework. Through such modification, a considerable amount of aluminum is also incorporated in the pores outside of the framework, where it adopts an octahedral coordination (ref. 16). The aluminum insertion can be obtained by reaction of silanol groups with AlCl3 or by the substitution reaction of silicon atoms of the framework by aluminum atoms provided as AlCl3 reagent. Strong acid sites including both Brönsted and Lewis acid sites were generated together with the acid sites of normal strength observed for ordinary ZSM-5 zeolite. (ref. 17)
AlOx coating of the pores of ultrastable zeolite Y (USY) has been proposed as a possible method for vanadium passivation of Fluid Catalytic Cracking catalysts. Two coating methods were presented: (i) the deposition of the [Al13O4(OH)24(H2O)12]7+ ([Al13]) complex from aqueous solutions and (ii) the anchoring of alumoxane by in situ triisobutylaluminum hydrolysis followed by calcination. (ref. 18)
Various ways for post synthesis incorporation of trivalent elements, such as aluminum, in ordered mesoporous materials have been reported. Post synthesis alumination of siliceous ordered mesoporous materials in principle offers advantages over the direct incorporation during synthesis. Aluminum often interferes with the ordering process during the formation process of the ordered mesoporous material. The accessibility to active aluminum sites incorporated during synthesis may be limited owing to pore blockage. In post synthesis incorporation of trivalent elements such as post synthesis alumination, the challenge is to obtain a well dispersed and uniform distribution of the trivalent elements such as aluminum over all mesopores of the material. Aluminum reagents when contacted with the ordered mesoporous silica tend to react and be deposited in the pore openings. The presence of long, mono-dimensional mesopores hinders the spreading of the trivalent element over the internal body of the material. Techniques which can uniformly disperse said trivalent elements, such as aluminum into the mesopores are of great interest (ref. 19).
In the literature, the preparation of aluminum-containing mesoporous MCM-41 materials was obtained by post-synthesis modification of a purely siliceous MCM-41 using different Al sources: AlCl3, aluminum isopropoxide and NaAlO2. The structure, thermal stability and acidity of these materials have been investigated and compared with Al-MCM-41 prepared by direct hydrothermal synthesis. Irrespective of the preparation method, the surface area, pore diameter, crystallinity and thermal stability of Al-MCM-41 decreased with increasing Al content. Post-synthesis modified materials possessed better thermal stability, and this method allows for the incorporation of more aluminum without disintegration of the mesoporous structure as compared to Al-MCM-41 prepared by direct hydrothermal synthesis. The post-synthesized Al-MCM-41 had a moderate acidity, comparable to that of the direct hydrothermally-synthesized material. (ref. 20)
The alumination of siliceous MCM-48 containing organic templates with sodium aluminate solution has been reported (ref. 21). The modification altered the porosity quite significantly.
Aluminium chlorohydrate solution which contains Al polycations was reported to be an efficient source of Aluminum for post-synthesis alumination of purely siliceous MCM-41. The material retained excellent structural integrity and showed enhanced acidity and catalytic activity. The amount of Al incorporated into the MCM-41 framework was dependent on the concentration of Al in the grafting solution; 27Al MAS NMR confirmed that a large proportion of the Al atoms was inserted into tetrahedral positions within the framework. TEM and XPS indicated that there were no separate surface alumina phases. The pore wall thickness increased with Al content, but pore size uniformity was maintained. Alumination generated Brønsted acid sites which increase in population as the Al content rose. Those materials exhibited considerable catalytic activity for cumene cracking and were superior to AlCl3-grafted MCM-41 or aluminum chlorohydrate grafted amorphous silica. (ref. 22)
O'Neil et al. investigated the use of supercritical solvents for post synthesis alumination of ordered mesoporous silica. O'Neil et al. prepared aluminum grafted MCM-41 material by reacting pure silica MCM-41 with aluminum isopropoxide in supercritical CO2 or propane. The supercritical fluid was shown to provide efficient transport of the aluminating agent into the mesoporous material. (ref. 19)
In another investigation, dry MCM-41 sample was dispersed in dry toluene containing various amounts of trimethylaluminum (TMA). The resulting mixture was maintained at room temperature for 48 h without stirring. Aluminum was found to be inserted into tetrahedral positions within the framework at room temperature. No further calcination was required. It was decided that TMA is an efficient aluminum source for the post-synthesis alumination. (ref. 23)
This literature overview teaches that incorporation of trivalent elements in mesoporous silicate-based materials, more in particular the alumination of said mesoporous silicate-based materials, can be achieved. The methods currently applied require the use of an aqueous medium, or of organic or supercritical solvents. The use of gaseous reagents such as AlCl3 appeared to be less successful.
Atomic Layer Deposition (ALD) Technique
Atomic layer deposition (ALD) is a process for depositing highly uniform and conformal thin films by alternating exposures of a surface to vapors of two chemical reactants (ref. 24). Atomic Layer Deposition (ALD) is used to deposit thin films with special features. The technology was originally developed for the fabrication of polycrystalline luminescent ZnS:Mn and amorphous Al2O3 insulator films for electroluminescent flat panel displays. Even though the ALD technology showed some benefits, the deposition rate was too low to make it economical at that time. Due to its complex surface chemistry, no real break-through involving ALD was achieved until 1985.
The decreasing device dimensions and increasing aspect ratios in the micro-electronics industry increased interest towards the ALD technique during the 1990s. ALD processes are attractive for a variety of applications in micro-electronics: deposition of diffusion barriers, dielectric films and electrodes for DRAM capacitors and thin dielectric films for gate stack applications. ALD has been used to deposit various materials, including several oxides, nitrides and pure metals. Since this is a layer-by-layer deposition technique it produces films of uniform thickness and excellent conformality.
Different from chemical vapor deposition (CVD), ALD technology is based on saturated surface reactions. The principle of ALD is based on sequential pulsing of chemical precursor vapors, which form about one atomic layer each pulse. This generates pinhole free coatings that are extremely uniform in thickness, even deep inside pores, trenches and cavities. (ref. 25)
The intrinsic surface control mechanism of the ALD process is based on the saturation of sequentially performed surface reaction between the substrate and precursor. The saturation mechanism makes the material growth rate directly proportional to the numbers of reaction cycles instead of the reactant concentration or time of growth.
The advantages of atomic layer deposition technique can be enumerated as follows:
1. Digital thickness control to atomic level (no rate monitor needed, just set the number of atomic layers).
2. Perfect 3D conformality, 100% step coverage: uniform coatings on flat, inside porous and around particle samples.
3. Large area thickness uniformity.
4. Potential for batch scalability (precursor sources are small and stacking of substrates is possible).
5. Pinhole free films, even over very large areas.
6. Excellent repeatability (wide process windows: many ALD processes are not very sensitive to temperature or precursor dose variations).
7. Low defect density.
8. Excellent adhesion due to chemical bonds at the first layer.
9. Digital control of sandwiches, heterostructures, nanolaminates, mixed oxides, graded index layers and doping.
10. Gentle deposition process for sensitive substrates (although a plasma may be used to enhance the deposition rate for certain processes, thermal ALD is often sufficient).
11. Low temperature deposition possible (RT-400C).
12. Atomically flat and smooth coating, copies shape of substrate perfectly.
13. Low stress because of molecular self assembly.
14. 100% film density guarantees ideal material properties (m, Ebd, k, etc).
15. Insensitive to dust (grows underneath dust!).
16. The deposition of thin films of a variety of materials has been reported (oxides, nitrides, metals and semiconductors).
17. The deposited film can be either amorphous or crystalline depending on the substrate and the deposition temperature.
18. Coatings have been reported on a variety of substrates (glass, plastics, Si, metals etc).
19. High production yields due to all these process benefits.
There are two fundamental self-limiting mechanisms in ALD: chemisorption saturation process followed by exchange reaction (CS-ALD) and sequential surface chemical reaction (RS-ALD).
The treatment of materials by ALD typically proceeds as follows.
First the material needs to be pretreated to bring the surface in a reactive state. This is typically carried out by heat treatment, although the use of a plasma may also be beneficial. Physisorbed molecules, most often water adsorbed from ambient air, are removed.
Step 1: Saturating reaction of a gaseous reactant (reactant 1, typically a metal compound) with the reactive sites on the support surface. The reaction is allowed to proceed until the surface is saturated with the adsorbing species and no more reaction takes place. Thereafter, excess reactant and possible gaseous reaction products are removed by an inert gas purge or by evacuation.
Step 2: Saturating reaction of another reactant (reactant 2, typically a non-metal compound) with the reactive sites on the support. The adsorbed species left behind by the first reactant form a major part of the reactive sites. Excess reactant and the gaseous reaction products are removed. The steps are schematically presented in FIG. 1 (ref. 26).
The atomic layer deposition technique has been applied in the area of catalyst preparation. Atomic layer deposition was used to prepare aluminum nitride on porous silica and alumina supports and to deposit catalytically active components on the obtained AlN/oxide supports. There are reports on aluminum nitride grown on porous silica by atomic layer deposition from trimethylaluminum and ammonia precursors. The ALD growth is based on altering, separated saturating reactions of the gaseous precursors with the solid substrate. The growth and the surface reactions were investigated by elemental analysis and solid state NMR measurements for 27Al and 29Si (ref. 26-27).
Active transition metal catalysts have been produced by ALD for a variety of purposes. They are listed in Table 1 (ref. 26).
TABLE 1Examples of reactions for which catalysts have been prepared byALD and description of the catalyst (ref. 26)ReactionCatalystSupportReactantsAlkane dehydrogenationCrOxAluminaCr(acac)3, aairVOxSilica, aluminaVO(acac)2, airEthene hydroformylationCoSilicaCo(acac)3, airToluene hydrogenationNiAluminaNi(acac)2, airCoSilicaCo(acac)3, airbCoAluminaCo(acac)2, airbAlkene metathesisWOxSilica, alumina,WOCl4 or WCl6, airmagnesiaMethane oxidationCoOxZirconiaCo(acac)3, airbMethanol oxidationTaOxSilica, alumina,Ta(OC2H5)5, airzirconiaVOxSilica, alumina,VO(acac)2, airzirconiaAlkene polymerisationCrOxSilicaCr(acac)3, airCrOxSilicaCrO2Cl2Alcohol dehydrationZrO2AluminaZrCl4, H2O
ALD is reported to be used for the modification of zeolite or molecular sieve membranes to decrease the effective pore size for molecular separation purposes. (ref. 28, ref. 40) Uniform nanostructured catalytic membranes had been fabricated by a combination of anodic aluminium oxidation and atomic layer deposition. The ALD method makes it possible to control pore diameters on the angstrom scale. (ref. 29)
Spatially controlled atomic layer deposition of ZnO, TiO2, V2O5 and Nb2O5 in anodized aluminum membranes have been reported (ref. 30). Conformal coating of metals over nanoporous AAO and SiO2 aerogels which can have applications in catalysis and gas sensors is known in the art. (ref. 31)
A plasma-assisted ALD process has been developed in which the ALD precursors are chosen to be nonreactive unless triggered by plasma, so that ALD can be spatially defined by the supply of plasma irradiation. Since plasma cannot penetrate within the internal porosity of mesoporous silica, ALD has been successfully confined to the immediate surface. This technique is useful for sealing of porous low dielectric constant films with a conformal layer and for progressive reduction of the pore size of mesoporous membranes. (ref. 32)
Several studies reported that ALD mediated grafting of vanadium and other elements, such as molybdenum and tungsten, on mesoporous silica by keeping the system in rigorous dry and unhydrated conditions resulted in superior catalytic performance in redox reactions (ref. 37-38).
Mahurin et al. (ref. 39) described the atomic layer deposition of TiO2 on mesoporous silica by ALD using TiCl4 and water reactants as a means to functionalize the mesoporous material. However, this will not significantly improve the activity of the mesoporous material in reactions demanding acid catalytic sites.